Process of making zinc oxide



1938 w. T. MAIDENS PROCESS OF MAKING ZINC 0XI'DE Original Filed Dec. 14,1933 grwc/wbo'n I amaze-MW 1m;

Patented Dec. 6, 1938 PATENT OFFICE PROCESS OF MAKING ZINC OXIDE WilliamT. Maidens, Columbus, Ohio, assignor to American Zinc, Lead & SmeltingCompany, St. Louis, Mo., a corporation of Maine Application December 14,1933, Serial No. 702,374 Renewed August 17, 1937 7 Claims.

' This invention relates to improvements in the manufacture of zincoxide and more particularly to methods for the manufacture of roundparticles of zinc oxide and controlling the size of such particles.According to one well known method for making zinc oxide, a mixture ofzinc ore and coal is spread upon a fiat grate through a series ofcharging doors of a suitable furnace A and at a high temperature(approximately 1400" C.). The zinc is reduced to metallic zinc andvolatilizedas a gas. As zinc vapor, diluted with the products ofcombustion, passes over the charges of coal and zinc ore it becomesoxidized through the action of air admitted to the furnace and, as italternately meets fresh amounts of zinc vapor and air, particles of zincoxide are formed. The relatively coarse pigment particles of zinc oxidefollow an irregular path through the flue and are finally delivered bysuction to a trail where they are cooled so that they can be safelyfiltered in a bag room.

In the operation of such a plant charges of coal and zinc ore aredelivered at periodic intervals to the grate, so that by the time thelast charge is inserted through the last door in the series, the ash orclinker is ready to be withdrawn from the point where the first chargeis inserted and the new charge is introduced. Under these circumstancesthe characteristics of the product formed are continually changing, andfurther, weather conditions and other factors beyond the control of thefurnace operation make it impossible to obtain a uniform product of thedesired quality.

According to the present invention, the furnace is operated under moreuniform conditions. The charges of coal and zinc ore are introduced intothe furnace as heretofore, but the draft of air to support combustion islimited and so regulated that the products instead of being a mixture ofzinc vapor and zinc oxide with C0, C02 and nitrogen, consist of no zincoxide and a ratio of carbon monoxide to carbon dioxide sufficient tomaintain zinc in the elemental state at the furnace temperature which ispreferably maintained at approximately 1200 C. This regulation of thedraft of air results in an atmosphere practically completelynonoxidizing to zinc vapor above the charge in the furnace. The gasesand zinc vapor under a slight positive pressure are then withdrawn fromthe furnace. In order to maintain the uniform flow of gas, it isdesirable to control the suction in the trail.

The temperature and composition of the gases can be held very uniformand this plays an important part in the uniformity of the zinc oxidemade by this process.

In the reduced state the gases from the furnace are passed to a suitabletower or chamber Where combustion takes place due to the introduction ofair into the current of gases on the way from the furnace to saidchamber. Preferably the passage of the gas from the furnace to thecombustion chamber is controlled to the end that the relative proportionof air and gas entering the combustion chamber may be so maintained thatan intense flame results. The air is introduced at this point by suctionor pressure at high velocity so that there will be considerableturbulence in the combustion chamber. When the furnace is operated underthese conditions, relatively large round particles of zinc oxide ratherthan needle shaped particles are formed. By round particles is meantthat no one dimension predominates, rather than that the particles arespherical. It is known in the art that when zinc oxide in the form ofneedles is used in the manufacture of rubber, the result is a stiffrubber, whereas the round particles make a soft rubber. When the furnaceis operated under the conditions of the present invention, relativelylarge, non-acicular particles are developed due to the growth of thesmall particles. The zinc oxide of this character although desirable forcertain round particlezinc oxide of fine particle size, and

prevent particle growth, water sprays are introduced into a tower orcombustion chamber and no other change is made; the air for oxidation isadmitted in slightly greater quantities than is theoretically necessaryto oxidize the zinc vapors as when coarse zinc oxide is being made. Byreason of the exothermic reaction when air is admitted there would be arise in temperature above that at which the gases leave the furnace butfor the excess of air admitted above that necessary to oxidize the zincvapors. This excess tends to neutralize the rise in temperature due tooxidation but is insufficient to lower the temperature of the combinedgas and zinc oxide particles below that at which the gases leave thefurnace, to wit, approximately 1200 C. In the tower, however, the watersprays at first somewhat lower the temperature (to approximately 1100C.) and subsequently the water sprays serve to still further reduce thetemperature to approximately 850 C. From the tower the gases are passedinto an adjoining chamber in which the temperature falls toapproximately 750 C., and the round particle size zinc oxide, togetherwith associated gases, pass from the second chamber into the trail onthe way to the bag room. Shortly after entering the trial thetemperature is decreased to approximately 650 C. These temperatures mayvary somewhat above or below those given, but are approximately asstated.

As pointed out above, this treatment results in small, fine, roundparticles of zinc oxide. Apparently, the water vapor even at the hightemperature indicated, is appreciably adsorbed by the zinc oxide whenthe water vapor is present in large amounts, producing a film on thezinc oxide particles that inhibits particle growth. Whatever may be thereason, the result, as indicated above, is particles of fine, round zincoxide. The amount of air added in making the fine particles is little,if any, more than that employed in making coarser particles, and it is,therefore, clear that the uniform fineness cannot be due to the actionof the air introduced.

Fine particle sized zinc oxide made by this process may carry a greateracidity than is desirable, and a second phase of the present inventionlies in the discovery that this method can be so modified as to makezinc oxide with unusual and desirable properties. It has beenuniversally believed heretofore that the refining of zinc oxide byreheating at 600 C. to 700 C. would unduly coarsen the particles, butwhen the fine zinc oxide particles of the present invention are thusreheated, there is obtained a product of medium acidity that is free ofunduly fine particles and also free of unduly coarse particles,resulting in remarkably uniformly sized zinc oxide particles.

Accordingly, the present invention further consists in refining theround particles of zinc oxide of the present invention by reheating,preferably in the presence of organic material whereby a product ofmedium acidity free from unduly fine particles and free from undulycoarse particles is obtained.

For the purpose of enabling the invention to be understood, reference ismade to the accompanying drawing, which is a diagrammatic view of aplant for making round particles of zinc oxides.

In the drawing, in which like reference numerals indicate like partsthroughout, I indicates a furnace with any suitably perforated grate,either stationary or travelling, as may be desired, and 2 indicatescharging openings for introducing coal and zinc ore into the furnace.The draft of air passing up through the grate for supporting combustionis regulated and controlled by any suitable means well known in the art.As here shown, the draft of air is regulated or controlled by means of afan 3 delivering air under low pressure to a conduit 4, and the passageof air from said conduit to the several sections of the furnace iscontrolled by suitable dampers 5. Care is taken to so regulate thesupply of air that the atmosphere above the grate shall be at all timesa practically non-oxidizing atmosphere, consisting of the products ofcombustion and the vaporized zinc. The temperature in this furnace mayvary somewhat above or somewhat below 1200 C. At 6 is indicated theusual or any suitable tower into which the fumes from the furnace passthrough an opening I, which is suitably controlled by baflies 8 tomaintain a degree of back pressure in the furnace. The mixed products ofcombus tion and zinc vapors pass from the tower 6 through a channel 9into a secondary chamber I from which chamber they are drawn out throughthe trail I I by means of a suction fan I2 and pass into the bag roomI3. The rate of movement of the vapors from the furnace and of the zincoxide particles and gases on the way to the bag room may be controlledby the speed of the fan I2, and if desired, by a suitable valve I4introduced in the trail.

As the products of combustion and zinc vapor pass from the furnace Iinto the tower 6, air is introduced under pressure at I5 under suchconditions as to produce considerable turbulence, the air admitted beingin somewhat greater quantities than is theoretically necessary tooxidize the zinc vapor but insufficient to lower the temperature belowthat at which the vapors leave the furnace.

This results in an intense flame. While in the tower 6 the gas issubjected to the action of water spray I6, and this water spray servesto somewhat lower the temperature of the combined gas and zinc oxideparticles so that they pass from the tower 6 to the channel 9 at atemperature of approximately 850 C, and thence into the chamber II],which chamber they leave at a temperature of about 750 C. and enter thetrail II where the temperature is still further lowered, so that theyenter the bag room I3 at a proper filtering temperature. The combustionof the zinc vapor is initiated in the tower 6 and the small roundparticles of zinc oxide are subjected therein to the action of the watersprays and then pass onward to the chamber I0 where the temperature issomewhat lower, and thence to the trail and to the bag room.

It will be observed that the temperature of the combined gas and zincoxide particles is progressively reduced on the way from the furnace tothe trail, and the zinc oxide particles are subjected to the action ofwater spray to arrest the increase in size of the particles, with theresult that the small, although not unduly fine particles of zinc oxideare secured.

As indicated above, the zinc oxide particles thus obtained may requiretreatment to control the acidity thereof, and for this purpose themedium size round particles of zinc oxide are refined by reheating,preferably in the presence of organic material, and by such refiningprocess there is obtained a product of medium acidity free from undulyfine particles and free from unduly coarse particles, thus producing aremarkably uniform sized fine particle zinc oxide of the desiredacidity.

It will be understood by those skilled in the art that various forms ofapparatus may be employed in the practice of the process, and that theinvention is not limited to the particular apparatus herein shown anddescribed. Any means for producing zinc vapor in a non-oxidizingatmosphere and carrying the same with a current of inert gas into acombustion chamber where the temperature is somewhat lowered and theparticles of oxide formed are subjected to water vapor, after which thetemperature of the oxide particles and the accompanying gases is slowlylowered on the way to the bag room, may be employed, the limits of theinvention being defined in the accompanying claims.

Having thus described the invention, what is claimed is:

1. The process of making zinc oxide of small non-acicular form whichconsists in vaporizing zinc in a reducing atmosphere, then rapidlymixing the zinc vapor with an oxidizing agent sufficient to oxidize butnot appreciably cool the vapor, and then subjecting the mixed vapor andoxidizing agent to the action of water spray.

2. The process of making fine particles size zinc oxide of mediumacidity, which consists in vaporizing zinc in a reducing atmosphere,then mixing the zinc vapor with sufficient air under turbulence tooxidize without appreciably cooling the vapor, then lowering thetemperature of the mixture with water spray, and then refining theproduct by heating. I

3. The process of making zinc oxide of small non-acicular form whichconsists in subjecting zinc ore to the action of burning coal in afurnace, so controlling the air draft to the furnace that the resultingzinc vapor is in a reducing atmosphere inthe furnace, then passing thevapor and reducing gases from the furnace and mixing them withsuificient air under turbulence to oxidize without appreciably coolingthe vapor and then spraying the mixture with water.

i. In a process for making zinc oxide in small particle form, the stepsof burning zinc vapor with air insufficient to materially cool the vaporand simultaneously spraying the burning vapor with water.

5. The process of making uniform size oxide of non-acicular form andmedium acidity which consists in vaporizing zinc in a reducingatmosphere at a temperature approximately 1200 C., passing the zincvapor and associated neutral gases into a combustion chamber, mixingsufficient air with said vapor and gases on their way to said combustionchamber to oxidize the vapor without materially cooling the same andthen spraying water into said combustion chamber, whereby thetemperature of the vapor and gases is reduced to approximately 1100 C.and the increase in the size of the zinc oxide particles is arrested,then passing the gases and oxide particles into a second chamber,maintaining the temperature in said second chamber at approximately 850C., then passing the zinc oxide and associated gases into a bag roomwhile gradually lowering the temperature, and then refining the zincoxide by reheating.

6. In a process of making zinc oxide of small non-acicular form, thesteps of burning zinc vapor with air without thereby materially reducingthe temperature, simultaneously spraying the burning vapor with water,and then passing the resulting zinc oxide to a bag room at a slowly andprogressively decreasing temperature.

7. The process of making zinc oxide, of nonacicular form which consistsin vaporizing zinc in a reducing atmosphere at a high temperature,introducing air under turbulent conditions and in an amount insufficientto lower the temperature, whereby the gases are held at a hightemperature for an appreciable period, and then spraying the gases withwater.

WILLIAM T. MAIDENS.

